Hydin seek: finding a function in ciliary motility

Abstract

One of the most surprising discoveries in cell biology in the past 5-10 years is the number of diverse human diseases that result from defects in ciliary assembly and/or motility, so-called ciliopathies (Badano, J.L., N. Mitsuma, P.L. Beales, and N. Katsanis. 2006. Annu. Rev. Genomics Hum. Genet. 7:125-148). The results presented by Lechtreck and Witman (see p. 473 of this issue) provide yet another example of how work in the model organism Chlamydomonas reinhardtii can reveal important insights into the underlying mechanisms of ciliary assembly/function and the diseases associated with defects in these organelles. By taking advantage of the wide array of experimental approaches C. reinhardtii offers, Lechtreck and Witman determined the precise axonemal location of hydin, a protein that, when mutated, causes hydrocephalus, and defined a unique role for hydin in ciliary motility.

Figure 1.

Transverse section of C. reinhardtii flagellar axoneme. The dark line represents the plane of bending. In the principle bend of the effective stroke, active sliding is generated by dynein arms on doublets 2–4 (green). In the principle bend of the recovery spoke, the generation of active sliding switches to doublets 6–8 (blue). When the bend initiates, the central apparatus is oriented parallel to the plane of bending, with the C1 tubule of the central pair oriented toward doublet 1 (Mitchell, 2003). However, as the bend propagates, the central apparatus rotates with a slight twist. Therefore, in C. reinhardtii, the orientation of the central apparatus changes relative to regions of active sliding. Based on the work of Lechtreck and Witman (2007), hydin (yellow; C2 central tubule) may play a role in regulating the switch between zones of active sliding.